An integrative investigation on the bump-on-tail ion distribution functions and nose spectral structures

Ions interacting with the magnetic and electric fields in the inner magnetosphere play an important role in the generation of waves. The unstable ion populations which contain free energy manifesting itself as a positive gradient region (also called bump-on-tail signature) in its distribution function can drive ULF waves in the low L-shells of magnetosphere. Ion nose structures shown in the particle energy-time spectrograms in the Earths inner magnetospheric observations are important signatures of inner magnetospheric ion access. Understanding the access of ions containing free energy to the inner magnetosphere and how ion nose structures are generated will provide insight into the physical dynamics of the inner magnetosphere. In this study, we use the Rice Convection Model (RCM) to simulate the dynamical evolution of ion distribution functions during substorm injections. The simulation demonstrates that the bump-on-tail distributions develop over a broad spatial region during the substorm expansion phase and can be explained as a result of velocity dispersion of protons injected into the ring current region. The injection of ions into the nightside magnetosphere leads to the unstable ion populations containing large amount of free energy which contribute to the nose-like structure in the inner magnetosphere. We will also discuss the spatial and temporal scales of the appearance of such structures.